The invention relates to a process and to a device for drying a liquid layer which has been applied to a carrier material moving through a drying zone and which contains vaporizable solvent components and non-vaporizable components.
Diverse drying processes and drying devices are used in the drying of large-area web-shaped goods to which liquid layers have been applied. Examples of typical goods to be dried are metal strips or plastic strips, to which liquid layers have been applied which, as a rule, are composed of vaporizable solvent components which are removed from the liquid film during the drying process, and of nonvaporizable components which remain on the carrier material after drying.
As a result of the coating, the surfaces of the carrier materials are provided with special properties, which only after the drying process are in the form which is desired for the later use. As an example of this, the coating of metal strips with light-sensitive layers may be mentioned, which are made up to give printing plates. The coating of metal strips or plastic films with substances in the form of a solvent-containing wet film, called liquid film in the text which follows, and the subsequent drying of the film thus represent a process which requires special installations in order to ensure the desired product quality of the layers. The essential point here is the process step of film drying, as the final process measure of coating.
In the drying of liquid films on carrier materials, it is usual to cause a heated gas, in particular air, to flow over the surface of the carrier materials in order to remove the solvent components from the film layer. The heated gas stream is here brought into direct contact with the liquid film, which has been applied in a uniform coating distribution to the carrier material which runs through a drying device. In order to ensure a streak-free and mottle-free dried film surface, i.e., a uniform distribution of the remaining components, the drying installations are fitted with devices which are intended to effect a favorable and/or uniform distribution of the air flow over the liquid film. This is intended to provide uniform drying across the entire width of the coated web. Furthermore, known drying installations have devices for minimizing disturbances in the air movements, which have an adverse effect on the film surface, partially due to turbulent flow movements, and cause mottling phenomena thereon.
A conventional construction of such a drying device comprises, according to U.S. Pat. No. 3,012,335, supplying, as uniformly as possible, the gas space directly above the liquid film which is to be dried with drier gas from a gas space, which is located along a certain length above the coating web, and which is supplied therewith, by means of a multiplicity of slots, nozzles, holes or porous solids. The continuously coated strip or coated plates on a revolving transport belt are here passed through the drying device continuously and with release of solvent vapor to the drier air. The drier air fed in can here be continuously renewed in open circulation or the air enriched with solvent can be discharged completely. A circulating-air process with partially renewed or discharged drier air can also be used.
Difficulties in the discharge of the drier air from the drying space are frequently caused by the fact that, in the case of longitudinal nozzles arranged transversely to the direction of running of the strip, or of longitudinal slots, a reduction in the nozzle outlet velocity occurs in the middle of the field, due to the pressure gradient in the case of lateral outflow, and hence the heat transfer and mass transfer transversely to the direction of running of the strip are also affected. The consequence thereof is overdrying of the edge, which causes undesired structuring of the dried films in many coating processes.
In the technical journal "Chemie-Ingenieur-Technik", Volume 42, No. 4 (1970), pages 927 to 929, Volume 43, No. 8 (1971), pages 516 to 519, and Volume 45, No. 5 (1973), pages 290 to 294, proposals are therefore made for optimizing the constructional design of nozzle fields in slot nozzle driers, which are intended to ensure constant heat transfer and mass transfer across the entire strip width of a drier. For optimizing slot nozzle driers, mass transfer measurements in impingement flow from slot nozzle fields with differing nozzle areas are empirically correlated within a wide range of the external parameters. The correlation found is used for determining optimum nozzle geometries with respect to the fan output per m.sup.2 of goods surface area. It is found here that a constant heat transfer and mass transfer across the strip width is achieved when the nozzle slots have slot widths which continuously increase from the strip edge towards the middle.
When large-area goods webs are dried, a high uniformity of the heat transfer and mass transfer across the strip width must frequently be demanded in order to avoid local overdrying and the associated reduction in quality. In these cases, slot nozzle fields are preferably used in which the slots are arranged transversely to the direction of running of the web. The edge overdrying observed here in the slot nozzle driers with outflow in the nozzle direction is to be ascribed to the distribution of the outlet velocity along the slots. In order to avoid this edge overdrying, it follows from this for nozzle driers, inter alia, that the outflow area should, as far as possible, be 3.5 times the nozzle outlet area, in order to obtain uniform drying across the width of the goods web.
It is now state of the art to carry out a contactless surface treatment in suspension driers for film strips or metal strips by means of a carrier air nozzle system (Journal "Gas Waerme International", Volume 24 (1975), No. 12, pages 527 to 531). The drier air enriched with solvent is here exhausted again directly in the nozzle fields, in order to eliminate the undesired transverse flow. This results in so-called nozzle driers or impingement jet driers, wherein above all the stagnation point-like flow of individual nozzles is a disadvantage, which tends, both in the laminar and the turbulent form of flow, to gas flow instabilities which inevitably lead to irreversible drying structures, particularly in the case of low-viscosity liquid films.
To avoid stagnation point-like flows in the initial region of the drier apparatus, the drier air is, according to PCT Application W082/03450, passed from an upstream chamber via suitable inlet orifices and flow baffles into a quietened intermediate chamber, from where a part of the drier air reaches the web, which is to be dried, via a porous filter element arranged in the immediate vicinity of the liquid film. The mode of action of such drying is based on the fact that, between the porous protective shield and the liquid film which is to be dried, a weak air flow is formed which is quietened but highly enriched in solvent and which is continuously renewed by exchange with the residual air flowing off transversely via the porous medium, so that, due to the relatively short overall length, pre-drying of the liquid film with a reduced tendency to mottling phenomena is achieved.
This type of drying is distinguished by predominant diffusion of the solvent vapor/air mixture through the porous protective shield, whereby complete drying-out of the liquid film becomes possible, in the almost complete absence of convective removal within the space between the strip and the protective shield only in the case of very great drier lengths or with the addition of down-stream auxiliary driers.
A particular disadvantage of the drying devices hitherto used is that, due to the solvent-laden air flows within the drying chamber, a sealing device compatible with the external atmosphere must be provided. Depending on the magnitude of the absolute pressure within the drier chamber directly above the liquid film, either, under vacuum conditions, a part of the required fresh air flows inwards via the finite sealing gap or, under positive pressure conditions, a part of the solvent-laden air flows outwards, and irreversible structures can be produced on the undried liquid film by the flow in the sealing gap.